1. Field of the Invention
The present invention relates generally to a solid-state imaging device using CCDs (Charge-Coupled Device), and more particularly to a solid-state imaging device having CCDs with an improved electrode structure.
2. Description of the Related
Recently, a solid-state imaging device using CCDs has been used as an imaging device in a video camera, an electronic still camera, etc. In this solid-state imaging device, photoelectric conversion storage sections for converting input light to electric charges and storing the charges are arranged two-dimensionally, and the stored signal charges are output through vertical CCDs and a horizontal CCD. The vertical CCDs and horizontal CCD have the structures described below.
FIG. 23A is a cross-sectional view of a vertical CCD of a conventional solid-state imaging device having a transfer structure of a single-layer electrode CCD, and FIG. 23B is a cross-sectional view of a horizontal CCD of the same imaging device. Reference numeral 1 denotes a silicon (Si) substrate 1, numeral 2 a first layer polysilicon electrode, and numeral 3 a silicon oxide film. The transfer electrodes of each of the vertical and horizontal CCDs are formed of single-layer polysilicon. When the single-layer polysilicon electrode structure is used for both the vertical and horizontal transfer-electrodes, a gap R (S) between transfer electrodes is normally designed to be a minimum dimension determined by the limit of lithography.
With respect to the layout of the solid-state imaging device, the length of each transfer electrode of the horizontal CCD must be made less than that of each transfer electrode of the vertical CCD. The length of each transfer electrode influences a potential profile at the time of charge transfer. In particular, in a horizontal CCD with short transfer electrodes, the potential profile departs greatly from an ideal one owing to a so-called short-channel effect. More specifically, electrodes 2b and 2c, as shown in the lower parts of FIGS. 23A and 23B, will now be considered by way of example. A potential between the electrodes 2b and 2c is influenced by a gap between the electrode 2b and the preceding electrode 2a and a gap between the electrode 2c and the following electrode 2d. Since the influence is greater in the horizontal CCD, a potential pocket PP occurs between its electrodes 2b and 2c.
The occurrence of the potential pocket in the horizontal CCD degrades a transfer efficiency of the horizontal CCD. In order to solve this problem, it is necessary to increase the driving voltage of the horizontal CCD. In other words, it becomes difficult to lower the driving voltage of the horizontal CCD.
FIG. 24A is a cross-sectional view of a vertical CCD of a conventional solid-state imaging device having a transfer structure of an overlapping double-layer electrode CCD, and FIG. 24B is a cross-sectional view of a horizontal CCD of the same imaging device. Reference numeral 4 denotes a second layer polysilicon electrode. The transfer electrodes of each of the vertical and horizontal CCDs are formed of overlapping double-layer polysilicon. The overlapping double-layer polysilicon structure is used for both the vertical and horizontal transfer electrodes in order to make the gap R (S) between transfer electrodes less than a minimum dimension determined by the limit of lithography, thereby to enhance a transfer efficiency between transfer electrodes, in particular, the transfer efficiency of the horizontal CCD. The gap R (S) between the transfer electrodes is determined by an interlayer insulating film provided between the first layer polysilicon and second layer polysilicon, and the gap R (S) is less than the minimum dimension determined by the limit of lithography.
In the double-layer Si electrode structure, a gap between polysilicon layers is generally filled with a polysilicon oxide film having a low withstand voltage. As a result, short-circuit occurs between polysilicon layers, and initial-stage defects and B-mode defects occur frequently. Consequently, the reliability of video cameras and still cameras using solid-state imaging devices is degraded. The gap between the transfer electrodes is made equal to the minimum gap limited by an electrode manufacturing process and a withstand voltage (leak-current, etc.).
Recently, there has been a demand for miniaturization of a solid-state imaging device in accordance with reduction in size and weight of cameras. At present the size of a photoelectric conversion storage section (or "cell") is 9 .mu.m. In future, this size is planned to be reduced to 5 .mu.m or less. In order to further enhance the transfer efficiency with such miniaturization of cell size, the gap between transfer electrodes will be decreased more and more.
However, if the gap between the transfer electrodes decreases, short-circuit between polysilicon layers occurs and the yield decreases. In each of the above-described prior art examples, the gap between the transfer electrodes of the horizontal CCD is equal to that of the vertical CCD. Since the area of the vertical CCD is much greater than that of the horizontal CCD, short-circuit between polysilicon layers occurs in the vertical CCD.
In order to obtain a low-resistance transfer electrode of a CCD, a polycide structure wherein polysilicon and silicide of a refractory metal are laminated may be adopted. In this case, the gap between transfer electrodes-in the silicide is equal to that in the polysilicon. Normally, after a pattern of transfer electrodes is formed by etching, post-oxidation is performed to increase a withstand voltage. However, since an oxide film of a silicide has a lower withstand voltage than an oxide film of a polysilicon, a withstand voltage across transfer electrodes lowers and the yield decreases.
In the prior art, if the inter-electrode gap is decreased to enhance the CCD transfer efficiency, as described above, short-circuit occurs between the polysilicon electrodes. Thus, the inter-electrode gap is set at a minimum gap limited by the electrode formation process and withstand voltage (leak current, etc.). This is a factor to prevent enhancement of transfer efficiency of CCDs (in particular, horizontal CCD). Besides, when the polycide structure is adopted in transfer electrodes of the CCD, the withstand voltage of the oxide film of silicide is low and the yield decreases. In addition, if miniaturization is advanced, the transfer efficiency of the horizontal CCD is degraded due to the short channel effect of the horizontal CCD.